In compact brake caliper units of rail vehicles, the wear of the brake linings and of the brake disk is compensated automatically by lengthening or shortening a push rod or pull rod adjuster. The lengthening or shortening of the actuator is carried out by means of a helical gearing as a nut-spindle system. In order to achieve the greatest possible adjustment per activation, the thread is embodied with a very large pitch. The thread is, therefore, not self-locking. While the one helical gear element, for example the nut, is secured in a rotationally fixed fashion, the other helical gear element, for example the spindle, is rotatably mounted. By means of an actuation mechanism, an actuator lever with a freewheeling mechanism (for example a sleeve freewheeling mechanism) is rotated, which actuator lever in turn rotates the spindle or the nut of the push rod actuator in such a way that the actuator is, for example, lengthened. The adjustment process is ended when the actuator lever abuts against a stop in the adjuster housing or when an axial force which can no longer be overcome by the actuation mechanism of the actuator builds up in the actuator, for example when the linings come to bear on the brake disk. Such an actuation mechanism is described, for example, in EP 0 732 247 B1, which is of the generic type.
In the version of a push rod adjuster which is disclosed according to FIG. 2 in EP 0 732 247 B1, the nut is secured against rotation in the yoke, and the spindle is rotatably arranged in a guide tube of the adjuster housing. For the purpose of adjustment, the actuator lever is actuated by means of a push rod with a ball head, which push rod engages in a dome of the actuator lever. The spindle is rotated along by means of the sleeve freewheeling mechanism which is pressed into the actuator lever. In this context, the wrap spring freewheeling mechanism slips through in the freewheeling direction. If the push rod releases the actuator lever again when the brake is released, the actuator lever is rotated back again into its home position by means of a restoring spring, in which home position the actuator lever abuts against a rigid stop of the adjuster housing. The sleeve freewheeling mechanism rotates here in the freewheeling direction and in the process the spindle is prevented from rotating along by means of the wrap spring.
EP 0 699 846 B1 also describes a disk brake for rail vehicles having a brake caliper and wear adjuster. In the embodiment as a push rod actuator which is described in the document, the actuator element is formed by an actuator sleeve with outer toothing (reference sign 61 in FIGS. 5 and 6). A sleeve freewheeling mechanism is pressed into this actuator sleeve. A gearwheel segment, by means of which the actuator sleeve can be rotated counter to the force of a restoring spring by means of a lever, is in engagement with the toothing of the actuator sleeve. The nut is entrained by means of the sleeve freewheeling mechanism, as a result of which the push rod adjuster is lengthened. However, the spindle is mounted in a rotationally fixed fashion in the actuator yoke. The restoring spring rotates the actuator sleeve and therefore also the gearwheel segment into the home position, and in the process the gearwheel segment abuts against a rigid stop on the adjuster housing.
In the case of a push rod adjuster, a rotation, for example of the spindle of the spindle-nut gear mechanism in a direction which shortens the push rod adjuster, is prevented by a wrap spring freewheeling mechanism between the spindle and the adjuster housing. This wrap spring freewheeling mechanism forms, in addition to the freewheeling mechanism of the actuator element, a further or second freewheeling mechanism. As a result of the wrap spring freewheeling mechanism, the spindle is held in its position when the actuator element is rotated back into its home position by the force of its restoring spring and/or when a large brake application force, and therefore a large torque, acts on the spindle during a braking operation. If such a wrap spring freewheeling mechanism is loaded by a torque, the mechanism experiences elastic deformation which results, on the one hand, from stretching lengthening of the spring wire and, on the other hand, from a wrapping orientation of the turns on the parts to be coupled, for example the spindle and adjuster housing. Under extreme conditions of use, oscillations which are applied from the outside can bring about an increase in the elastic stretching in that they locally reduce the coefficient of friction among individual turns and as a result more turns are involved in the transmission of torque. If the wrap spring freewheeling mechanism under load has an excessively large elastic deformation or degree of stretching in the locking direction, the torque which acts in the helical gearing has to be partially or even entirely supported via the sleeve freewheeling mechanism of the actuator element if the actuator element is in its home position on the stop of the adjuster housing. It is then not possible to rule out damage to the sleeve freewheeling mechanism and therefore degrading of the function of the wear adjuster.